This invention relates to a method for improving energy and solvent recovery in various industrial evaporation processes wherein a volatile component is vaporized from a wet material and carried away by contact with a hot, relatively dry gas stream. More particularly, the invention is concerned with the drying of moist solids or semi-solid pastes and with the concentration of dilute liquors, both of which are common, energy-consuming processes.
Most industrial evaporation is done by passing heated gases (often air or nitrogen) through or around the material to be dried, and then venting the warm, moist mixture of gases and vapors to the atmosphere. The energy for heating the solid and evaporating the liquid contained therein may be supplied in one of several ways. For example, the gaseous drying medium may be preheated in a heat exchanger (frequently, one in which process steam is condensed) located upstream of the dryer, so that the energy required in the dryer is carried in as the sensible heat of the hot drying gases. Alternatively, the heat exchanger may be located in the dryer itself (particularly convenient in the case of a fluidized bed dryer) so that the heat may be transferred directly to the material being dried. In such processes, the large quantities of heat required to evaporate the water or solvent are lost in the form of the sensible and latent heats of the vented gas/vapor mixture. As a result of this loss of energy, the cost of energy makes up a large percentage of the total cost of drying and concentration operations, and this cost is increasing due to the ever-rising cost of fuel.
Mixtures of vapors and noncondensable gases are also produced in processes which concentrate dilute liquid solutions by evaporation. It is frequently uneconomical to concentrate a solution simply by boiling off the excess solvent, because this requires relatively high-grade heat. Moreover, the higher temperatures often required to boil solvent from a liquid mixture may be unacceptable if a solute is heat-sensitive. In such cases, evaporation and concentration may be effected at a relatively low temperature if a gaseous stream (frequently air or nitrogen) is used to reduce the partial pressure of solvent vapors in the drying atmosphere and to sweep the volatile component from the evaporator. For example, aqueous process waste streams are concentrated in air-blown packed-tower concentrators, the energy required for water evaporation being obtained by the condensation of steam and by the transfer of this energy to the air stream or to the liquor being concentrated. As in the case of a solids drying process, the moist mixture of gases and vapors exiting the concentrator is frequently exhausted to the atmosphere, resulting in losses of materials and of significant amounts of sensible and latent heat, which losses the method of the present invention can substantially reduce.
Another disadvantage of conventional drying or concentration processes is that discharge to the atmosphere of vapors, particularly those of organic solvents, may be unacceptable on economic or environmental grounds. The method of the present invention permits the recovery not only of the solvent vapors themselves but also of the heat contained therein.
The drying of solids and of semi-solid materials such as pastes and the concentration of dilute liquors thus share a number of common features: (1) the basic operation is one of evaporating a liquid; (2) evaporation is frequently assisted by the use of a relatively hot, gaseous drying medium that serves to carry the volatile component out of the dryer or concentrator and, in some cases, to carry the energy required for evaporation into the dryer or concentrator; and (3) a warm mixture of drying gases and vapors is produced that is often exhausted to the atmosphere. Such processes share two major problems: (1) the sensible and latent heat of the gas/vapor mixture is lost upon venting this mixture, resulting in high energy costs; and (2) both the gaseous drying medium and the solvent vapor are simultaneously lost, resulting in high materials costs (except where the drying medium and vapor are air and water vapor, respectively).
One evaporation technique known in the prior art to provide an opportunity for heat recovery is vacuum drying or concentration. Here there is no air to be heated, and the water vapor removed can be compressed to a convenient pressure and condensed in a heat exchanger that returns the heat of condensation to the material being dried or concentrated. The primary disadvantage of such a system is that the material to be dried must be transferred in and out of a vacuum chamber, a difficult task to accomplish at reasonable cost with solid materials. Furthermore, the vacuum dryer itself is a costly piece of equipment since it must often be large and capable of operating at pressures substantially below atmospheric pressure. Because vacuum operation can be problematic, the drying or concentration process is frequently conducted at near-atmospheric pressure, assisted by the supply of a dry gas stream that carries vapors from the dryer and concentrator. Unfortunately, this mode of operation complicates the task of energy and solvent recovery.
A number of methods have been proposed for the recovery of heat from moist or solvent-laden gas and vapor mixtures arising in industrial solids drying and liquor concentration processes. Winstel, in U.S. Pat. No. 4,028,817, describes a heat exchanger-type heat recovery apparatus useful in industrial laundry dryers wherein a transfer of predominantly sensible heat from the hot dryer exhaust gases to fresh, substantially dry air is effected; however, since the vapors in the exhaust mixture are not condensed, their latent heat is not recovered and the vapors are exhausted to the atmosphere. Kulling, in U.S. Pat. No. 4,145,818, discloses a process for the removal and recovery of vaporized liquids from gases in fluidized-bed drying operations that involves cooling a portion of the exhaust gas stream in order to effect the condensation and recovery of vapors present in that stream; however, means for recovering the latent heat contained in the vapors are not provided. Erikson, in U.S. Pat. No. 3,131,035, discloses a three-step process for extracting waste heat from the exhaust gases of a direct-fired dryer or concentrator comprising the steps of exhaust gas scrubbing, a first heat exchange to recover part of the heat contained in the exhaust gases for use elsewhere in the same or another process (e.g., in concentrating a liquor under subatmospheric pressure), and a second high-temperature heat exchange against hot furnace gases for the purpose of preheating the now relatively dry waste gases prior to their incineration in the dryer furnace. Rothchild, in U.S. Pat. No. 4,150,494, teaches the recovery of both the latent heat contained in a vaporized solvent as well as the vapor itself in a process wherein inert nitrogen gas is generated to blanket the material being dried (in this case, a solvent-borne coating on an object). Although the latent heat of the solvent is indeed recovered in this process, it is recovered at a very low effective temperature by using it to vaporize the liquid nitrogen which is the source of the blanketing gas.
Another method of accomplishing heat and solvent recovery is to use a heat pump to refrigerate and dry the exit gases, condensing the vapors in the process and returning the heat removed to the dried, recirculating gases. Stevens, in U.S. Pat. No. 4,134,216, discloses a product drying apparatus for particulate materials based on this principle, and Mehta, in U.S. Pat. No. 4,247,991, describes a process wherein one or more heat pumps in combination with a desiccant bed are used to recover energy and moisture, respectively.
Semipermeable media have also been employed for the recovery of vapors from mixtures with gases. For example, Booth, in U.S. Pat. No. 3,420,069, describes a condensor-separator in which a heat exchanger constructed from porous sintered metal tubes is used to remove condensed and entrained liquids from gas streams, although without significant heat recovery. Ketteringham and Leffler, in U.S. Pat. No. 3,511,031, describe a similar means for dehumidifying air in an enclosed space such as the cabin of a spacecraft by condensing water vapor in pores of microporous membranes, again without heat recovery. Finally, Arnold, in U.S. Pat. No. 3,811,319, describes a membrane gas separator system suitable for the removal of condensable organic materials and, in some cases, of water from gaseous and liquid mixtures for the purpose of permitting analysis of the organic material in the mixture by mass spectrometry or other analytical technique. Only the first of these three patents is concerned with industrial drying, however, and none discloses significant recovery of the latent heat of the vapor for reuse.
Still other techniques have been used to reduce the energy requirements of evaporative processes for the concentration of liquid mixtures or solutions. For example, multieffect evaporation (as discussed on pages 785-790 of King, Separation Processes, 2nd edition, 1980) is based on transfer of the latent heat of the vapors generated in one effect to the solution being concentrated in another effect that is operated at a lower pressure and, therefore, at a lower temperature than the first effect. This transfer of energy is accomplished in a heat exchanger that condenses vapors from the first effect and simultaneously heats the liquid in the second effect. Closely related to multieffect evaporation is vapor-compression evaporation, wherein the overhead vapors from the concentrator are compressed and then subsequently condensed in an indirect heat exchanger in contact with the liquid being concentrated. Vapor-compression evapdration is discussed, for example, by Holiday in Chemical Engineering 89(1982)118. These processes have been applied, for example, to the concentration of brines and the accompanying production of potable water. While both multieffect evaporation and vapor-compression processes are economical of energy, these processes suffer from the limitation that evaporation must take place at the boiling temperature of the liquid being concentrated, since the use of a gas stream to reduce the temperature of evaporation and to carry away vapors is precluded.
To summarize, known industrial processes for the drying of solids and semi-solids and for the concentration of liquid mixtures and solutions frequently benefit from the use of a relatively hot, gaseous drying medium which serves to carry the volatile component out of the dryer or concentrator and, in some cases, to carry the energy required for evaporation into the dryer or concentrator. The warm mixture of drying gases and vapors which is produced is often exhausted to the atmosphere. The recovery of the energy contained in the gaseous and vapor components in such exhaust mixtures and the recover of the components themselves are long-standing and important problems that remain to be solved.
It is, therefore, an object of the present invention to provide a method for recovering a significant fraction of the sensible and latent heat present in moist or solvent-laden dryer and concentrator exhaust vapors in order to permit the recycle and reuse of that energy in the drying or concentration process and thereby improve the energy efficiency of the evaporation process to an extent and in a manner not previously contemplated.
It is a further object of the present invention to accomplish the efficient recovery of water or solvent vapors present in dryer or concentrator exhaust streams in order to reduce losses of valuable solvents and minimize the environmental impact of their discharge.
Still another object of the present invention is to separate and recover relatively dry gas from the dryer or concentrator exhaust in order to permit its reuse, thus saving the sensible heat contained therein, minimizing the expense of providing fresh gas (particularly important where the gas is more expensive than air), and minimizing environmental pollution. Recycle of the relatively dry gas stream also minimizes the environmental impact of its discharge, since it will generally contain residual amounts of solvent as well as other gaseous and toxic or odiferous components arising in the drying or concentration process.
These and other objects are accomplished by the present invention, which is summarized and described below.